staying "on the grind" means to work hard, always be hustling, or otherwise engaged in money-making or woman-procuring activities.

people who were once poor and become wealthy might then become lazy and indolent and spend all day in champagne-filled jacuzzis whereas other newly-successful people might continue to work hard, thus "staying on the grind"

there are many examples from 50 Cent's latest release, "the Massacre", but i'll offer up two:

excerpt from "God gave me style":

A O.G. told me God's favorites have a hard time
You out the hood, that's good now stay on the grind

Chorus from "Bitch get in my car":

I got no pickup lines
I stay on the grind
I tell the hoes all the time
Bitch get in my car (Bitch get in)
I got my 64, ridin' on Dayton spokes
And when I open that do'
Bitch get in my car

2) Grind it Out, on the grind: working particularly hard, like when experiencing
an unlucky run of cards, to make an acceptable return on your time invested;
or when making a solid return when playing from a small bankroll at lower limits

Poker Pro's: Mugsy Russo is on the grind , The grinder Michael Mizrachi, Phil Ivey is grinding it out on the tournement circuit

"He Quit his job to become a grinder , he just won the world poker tour"

A term used to describe the act of playing Call of Duty. However, in order to be truly on the grind, you must not only experience the insatiable urge to do work, but also run shit for at least 2 hours. You cant be on the grind, or grinding, if you play any less. And for most of us, we'll stay on the grind a lot longer.

(a)"You off work yet?"
(b)"Yeah, just got home."
(a)"You on the grind?"
(b)"You know it, been murkin scrubs."

(a)*cough cough cough*-"Damn that shit is fire"
(b)"Time to get on the grind"
(a)"No doubt. The sticks are calling"